The present invention relates to plasma polymer membrane for the separation of one component from an organic feed. In a preferred embodiment the separation is aromatics from saturates.
The use of membranes to separate aromatics from saturates has long been pursued by the scientific and industrial community and is the subject of numerous patents.
U.S. Pat. No. 3,370,102 describes a general process for separating a feed into a permeate stream and a retentate stream and utilizes a sweep liquid to remove the permeate from the face of the membrane to thereby maintain the concentration gradient driving force. The process can be used to separate a wide variety of mixtures including various petroleum fractions, naphthas, oils, hydrocarbon mixtures. Expressly recited is the separation of aromatics from kerosene.
U.S. Pat. No. 2,958,656 teaches the separation of hydrocarbons by type, i.e., aromatic, unsaturated, by permeating a portion of the mixture through a non-porous cellulose ether membrane and removing permeate from the permeate side of the membrane using a sweep gas or liquid. Feeds include hydrocarbon mixtures, e.g., naphtha (including virgin naphtha, naphtha from thermal or catalytic cracking, etc.).
U.S. Pat. No. 2,930,754 teaches a method for separating hydrocarbons, e.g., aromatic and/or olefins from gasoline boiling range mixtures, by the selective permeation of the aromatic through certain non-porous cellulose ester membranes. The permeated hydrocarbons are continuously removed from the permeate zone using a sweep gas or liquid.
U.S. Pat. No. 4,115,465 teaches the use of polyurethane membranes to selectively separate aromatics from saturates via pervaporation.
The principle in the separation of aromatics and saturates in the above polymer membranes is that in a feed consisting of a mixture of aromatics and saturates the aromatic molecules are preferably soluble in the membrane compared to the saturated molecules and, as a result, the aromatics are preferentially removed at the permeate side of the membrane. In the case of polyurethane membranes, the polymer consists of a hard block-soft block copolymer. The soft blocks preferentially sorb aromatics while the hard blocks act as effective crosslink to provide the mechanical stability for the membrane. The soft blocks usually consist of polyethylene adipate oligamers in which the carbonyl groups are responsible for the preferential sorbtion of the aromatics.
In addition to its solubility of the molecule, the diffusion coefficient also plays a role in determining the selectivity of the membrane. This is because the flux F.sub.i of molecular species i through the membrane is given by: EQU F.sub.i =D.sub.i (dC.sub.i /dx)
where D.sub.i is the diffusion coefficient, C.sub.i is the concentration in the membrane and dC.sub.i /dx is the concentration gradient. Thus the flux of the molecules is governed both by the solubility as well as by the diffusion coefficient.